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First Report of Leaf Spot of Rudbeckia hirta var. pulcherrima Caused by Septoria rudbeckiae in Korea

Rudbeckia hirta L. var. pulcherrima Farw. (synonym R. bicolor Nutt.), known as the black-eyed Susan, is a flowering plant belonging to the family Asteraceae. The plant is native to North America and was introduced to Korea for ornamental purposes in the 1950s. In July 2011, a previously unknown leaf spot was first observed on the plants in a public garden in Namyangju, Korea. Leaf spot symptoms developed from lower leaves as small, blackish brown lesions, which enlarged to 6 mm in diameter. In the later stages of disease development, each lesion was usually surrounded with a yellow halo, detracting from the beauty of the green leaves of the plant. A number of black pycnidia were present in diseased leaf tissue. Later, the disease was observed in several locations in Korea, including Pyeongchang, Hoengseong, and Yangpyeong. Voucher specimens were deposited at the Korea University Herbarium (KUS-F25894 and KUS-F26180). An isolate was obtained from KUS-F26180 and deposited at the Korean Agricultural Culture Collection (Accession No. KACC46694). Pycnidia were amphigenous, but mostly hypogenous, scattered, dark brown-to-rusty brown, globose, embedded in host tissue or partly erumpent, 50 to 80 μm in diameter, with ostioles 15 to 25 μm in diameter. Conidia were substraight to mildly curved, guttulate, hyaline, 25 to 50 × 1.5 to 2.5 μm, and one- to three-septate. Based on the morphological characteristics, the fungus was consistent with Septoria rudbeckiae Ellis & Halst. (1,3,4). Morphological identification of the fungus was confirmed by molecular data. Genomic DNA was extracted using the DNeasy Plant Mini DNA Extraction Kit (Qiagen Inc., Valencia, CA.). The internal transcribed spacer (ITS) region of rDNA was amplified using the ITS1/ITS4 primers and sequenced. The resulting sequence of 528 bp was deposited in GenBank (Accession No. JQ677043). A BLAST search showed that there was no matching sequence of S. rudbeckiae; therefore, this is the first ITS sequence of the species submitted to GenBank. The ITS sequence showed >99% similarity with those of many Septoria species, indicating their close phylogenetic relationship. Pathogenicity was tested by spraying leaves of three potted young plants with a conidial suspension (2 × 10⁵ conidia/ml), which was harvested from a 4-week-old culture on potato dextrose agar. Control leaves were sprayed with sterile water. The plants were covered with plastic bags to maintain 100% relative humidity (RH) for the first 24 h. Plants were then maintained in a greenhouse (22 to 28°C and 70 to 80% RH). After 5 days, leaf spot symptoms identical to those observed in the field started to develop on the leaves inoculated with the fungus. No symptoms were observed on control plants. S. rudbeckiae was reisolated from the lesions of inoculated plants, confirming Koch's postulates. A leaf spot disease associated with S. rudbeckiae has been reported on several species of Rudbeckia in the United States, Romania, and Bulgaria (1–4). To our knowledge, this is the first report of leaf spot on R. hirta var. pulcherrima caused by S. rudbeckiae in Korea.

References: (1) J. B. Ellis and B. D. Halsted. J. Mycol. 6:33, 1890. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ February 2, 2012. (3) E. Radulescu et al. Septoriozele din Romania. Ed. Acad. Rep. Soc. Romania, Bucuresti, Romania, 1973. (4) S. G. Vanev et al. Fungi Bulgaricae 3:1, 1997.